1. Field of the Invention
The present invention relates to a biaxial oriented polyester film and a method of manufacturing the same, and to a photographic substrate and a photographic light-sensitive material using the biaxial oriented polyester film. The biaxial oriented polyester film is suitable for use in various applications, in particular as a photographic substrate.
2. Description of the Related Art
Polyester films have excellent physical and chemical characteristics and have been used in various fields. Among these polyester films, a biaxial oriented polyester film, which has been stretched longitudinally and transversely so as to have high strength, is excellent in electric characteristics, chemical resistance, mechanical characteristics, flatness, dimensional stability, and economic efficiency, and therefore has been used in a large number of fields such as photography, magnetic recording media, and packaging.
The biaxial oriented polyester film has a drawback in that the film is not as good in slidability as in flatness, and this drawback adversely affects handling of the film in the manufacturing and working processes. Moreover, when the biaxial oriented polyester film is used as a substrate for a magnetic recording medium such as a photographic film (i.e., a photographic light-sensitive material) or a magnetic tape, excellent slidability is required so as to ensure travelling stability of the film in a camera, a developing device, a magnetic recording/replaying device, or the like.
In order to respond to such a demand for improvement in slidability, there are proposed a method of adding inorganic fine particles such as silica or calcium carbonate to the biaxial oriented polyester film, and a method of depositing fine particles containing calcium, lithium or phosphorus in the polymeric system at the time of synthesis of the polyester. With these methods, when the biaxial oriented polyester film is formed, projections are formed on the surface of the film by fine particles, and slidability is thereby obtained. However, since various apparatuses have been miniaturized and made precise recently, further improvement in slidability is desired.
Increasing the particle diameter of the fine particles to be added and the amount of the fine particles used is effective for improving slidability. However, scratches may be formed on the surface of the film by a roll during stretching of the film in the longitudinal direction thereof. Therefore, considering the suitability thereof for the longitudinal stretching of the film, the use of fine particles having a small particle diameter is desired, and the use of an excess of fine particles must be avoided. Further, a high degree of transparency is required in a substrate for a photographic film (a photographic substrate). However, a problem arises in that the transparency of the film decreases when fine particles having a large particle diameter are used or a large amount of fine particles are used.
Namely, in the biaxial oriented polyester film, an improvement in slidability is traded off against an improvement in suitability for the longitudinal stretching of the film and in transparency. Therefore, a biaxial oriented polyester film satisfying all of these characteristics and a manufacturing method thereof have been desired.
Accordingly, an object of the present invention is to provide a biaxial oriented polyester film and a method of manufacturing the same, in which a sufficient suitability for the longitudinal stretching and a sufficient transparency are achieved while a high degree of slidability is ensured. Further, another object of the present invention is to provide a photographic substrate and a photographic light-sensitive material utilizing the biaxial oriented polyester film having the above-described excellent characteristics.